The Light Stuff

Tiered grow lights are a space-efficient way to produce lots of vegetable, herb, and flower transplants for spring gardens.

Light. The word is so common we often don't think about what it is. For green plants, light is the engine that drives growth. Sunlight is the best light for that growth, but almost since the invention of electricity, scientists and gardeners have used artificial light to grow and study plants. Artificial light has the great advantage of being controllable. Whereas natural light might be too intense (bright sun) or weak (heavy clouds) to provide the ideal light for delicate seedlings, you can fine-tune the intensity and duration of artificial light.

For plants, fluorescent light is the simplest and easiest artificial light to use. Short of a greenhouse, a fluorescent light setup is the best way to start seeds or root cuttings indoors. Fluorescent light gardens are ideal for growing and blooming compact plants like African violets, some orchids, and many other indoor plants. While this article focuses on fluorescent lights, I've added some information about High Intensity Discharge (HID) lights for plants that require more light intensity to flourish indoors. Hydroponic growers use HID lights to produce tomatoes, peppers, lettuce, and other plants indoors year-round. So, let's talk about lights for your indoor garden.

Fluorescent Lights

These are the most common, most efficient, and most cost-effective lights gardeners can use to grow plants. They give more light per watt of electricity with less heat than common incandescent lights. By virtue of their light quality and cool operating temperature, fluorescent lights are by far the preferred way to establish seedlings.

Fluorescent tubes range in power from 15 to 215 watts. Most useful for indoor gardeners is the 4-foot-long, 40-watt "bipin" tube, with two contact points at each end. In general, the cost of a tube varies according to how common it is. For instance, a 4-foot cool- or warm-white tube costs as little as $2, while a full-spectrum tube specialized for plants costs more than $20. There are also energy-efficient T-8 and T-12 fluorescent light tubes. These bulbs are more expensive, but they provide a greater amount of full-spectrum light than conventional fluorescent bulbs, they last longer, and they use up to 20 percent less electricity. These new, energy-efficient tubes also require special bulb fixtures.

Moth orchids are one of the easiest orchids to grow under fluorescent lights.

Tubes' useful lifetimes are surprisingly variable, ranging anywhere from 12,000 hours (about 18 months) to 34,000 hours (almost four years). Usually the least expensive cool-white tubes are the shortest-lived. Replace all tubes before they burn out, when they've reached about 70 percent of their rated life. For example, replace a tube used 14 hours a day and rated at 20,000 hours after about 33 months.

Light Quality

Fluorescent tubes vary in the color of light they produce. While some artificial lights approach the full spectrum of wavelengths found in sunlight, none duplicate it. In all cases, artificial lights produce more or fewer wavelengths than sunlight. But plants don't need every color found in natural sunlight to grow and flower well. Plants mostly use wavelengths at the two ends of the visible spectrum: the blue end for foliage growth and the red end for flowering and fruiting.

Cool-white bulbs produce an abundance of blue light, but little red light. The so-called warm-white tubes complement them by producing more red light. Many established plants grow well under a combination of cool-white and warm-white tubes. But for proper development seedlings need brighter light as soon as they germinate; they require full-spectrum fluorescent tubes specifically designed for plant growth.

Light Intensity

Some light setups, such as this one from Mr. Light, can double as plant stands, maximizing the space plants take up in your home.

It stands to reason that the brightest lights deliver the most energy to plants. But light intensity is about the quantity of light reaching plants, not the brightness of that light. While most fluorescent lights may appear similarly intense, careful measurement reveals many differences. The most basic — and generally most useful — measurement of light intensity is a simple calculation of watts of fluorescent light per square foot.

Most vegetables and flowering plants need 25 to 30 watts of fluorescent light per square foot. Houseplants and seedlings do well with 15 to 20 watts, while germinating seeds need just 10 to 15 watts per square foot. A standard 4-foot-long, 40-watt fluorescent tube provides 10 watts per square foot. Two 40-watt tubes spaced six inches apart supply double that amount. Therefore, consider carefully what size light fixture to use. Assuming that most fixtures are about a foot wide, the intensity of light from a four-tube fixture (40 lamp watts per square foot) will be double that of one holding just two tubes (20 lamp watts).

Light produced by fluorescent tubes diminishes rapidly over distance. Most plants do best when their leaves are within six inches of the light tubes. The light intensity six inches below two 40-watt, cool-white tubes is about 700 foot candles. African violets grow well given 600 foot candles for 18 hours a day, so tubes six inches above them are just right. Seedlings need more light intensity, so position lights two to three inches above them, close enough to deliver about 1,000 foot candles to the leaves. Seedlings need 14 to 16 hours of fluorescent light a day. Given insufficient light, seedlings become leggy and may topple.

HID Lights

To grow fruiting crops (such as tomatoes) or flowers that require more light to bloom (such as some orchids), you need High Intensity Discharge (HID) lights. While larger, more expensive, and less efficient than fluorescents, HID lights pack a light punch up to 1000 watts. There are two basic types of HID light: metal halide and high-pressure sodium. Metal halides provide strong blue-spectrum light waves and are best for plants that don't need to flower to grow to maturity, such as lettuce and herbs. Sodium lamps provide more red light waves and are good for flowering and fruiting plants, such as tomatoes. As with cool- and warm-white fluorescent bulbs, it's best to combine HID lights to provide the full spectrum of light waves needed for proper plant growth. Some newer lamps on the market produce both red and blue light waves.